Modelling the efficacy of entomopathogenic nematodes in the regulation of invertebrate pests in glasshouse crops

Abstract

Summary1. Entomopathogenic nematodes belonging to the families Heterorhabditidae and Steinernematidae are lethal obligate parasites of a wide range of invertebrate species. These nematodes exhibit many characteristics that make them ideal candidates as biological control agents of insect pests (rapid host death, high reproductive rates, easily mass-reared in vitro, easy application techniques). 2. However, at present, the number of pest species to which these nematodes are applied successfully is small. Clearly, there is a need to develop existing knowledge of the nematode into a more complete understanding of the nematode±pest system as a whole. 3. To consider the potential of entomopathogenic nematodes as biological control agents, we adopted a generalized analytical modelling approach and, using realistic parameter estimates, determined the conditions under which these nematodes can regulate a pest population. 4. Stability analyses suggested that entomopathogenic nematodes may not be capable of regulating a host population to a stable equilibrium. Long-term persistence of the host and nematode population is unlikely, due to the highly destabilizing eects of the parasite±host relationship. As such, these nematodes may be better suited to short-term control through inundative application techniques rather than long-term regulation. 5. This preliminary generalized model highlights areas where further work is needed. This includes estimation of the probability of nematode infection in the ®eld, the eect of host size on the transmission cycle and the in¯uence of spatial heterogeneity on stability.

Evaluating the Efficacy of Entomopathogenic Nematodes for the Biological Control of Crop Pests: A Nonequilibrium Approach

“…The original analysis by Fenton et al (2000) described this life cycle by means of a simple susceptible-infected model, in which the pest and nematode populations were assumed to have continuous, overlapping generations. However, in this article, we are dealing specifically with short-term control, so the highly generalized model may mean that important transient dynamics of the system are lost.…”

“…In a previous article, we used a simple, generic model to explore the use of these nematodes in biological control (Fenton et al 2000). The analyses suggested that the inability of nematodes to persist as control agents might be due to severe instabilities inherent in the system.…”

“…Nematode mortality rates in the field (m) are system specific, depending on substrate type and abiotic factors (see Fenton et al 2000 The major problem with validating host-pathogen models is the lack of information on the rate of transmission (b). However, Scheepmaker et al (1997) provide information on sciarid fly abundance on four sampling dates (days 3, 10, 24, and 38) following nematode application.…”

“…As such, entomopathogenic nematodes can be thought of as a generalizable model, allowing insight into other hostpathogen systems. Hence, much of what follows can easily be extrapolated to consider microparasites and parasitoids.In a previous article, we used a simple, generic model to explore the use of these nematodes in biological control (Fenton et al 2000). The analyses suggested that the inability of nematodes to persist as control agents might be due to severe instabilities inherent in the system.…”

“…We therefore resort to a simulation approach to identify the key parameters determining the rate of control. All parameter values were based on those recorded in the literature (see Fenton et al 2000 for a review), and we assumed an initial density of 10 adult pests per unit area invaded at time . On t p 0 day 20, a single dose of 10,000 nematodes per unit area was applied, which either died at a constant rate m or infected pest larvae at rate b. Simulations were run for 100 d following application, and the time taken for the pest population to be suppressed to an arbitrarily low density (in this case, 0.1 per unit area), below which extinction was assumed to have occurred, was recorded.…”